This invention relates to oxidative coupling agents or catalysts for the conversion of methane to ethane, ethene and higher hydrocarbons having a greater molecular weight than methane.
In the search for petroleum, large amounts of natural gas are discovered in remote areas where there is no local market for it. The dominant technology now employed for utilizing remote natural gas involves its conversion to syngas, a mixture of hydrogen and carbon monoxide. While syngas processes fulfill the need for an easily transportable liquid that can be converted to several useful products, it is an expensive intermediate. Oxygen is added to the rather inert methane molecule which is advantageous when products such as methanol or acetic acid are desired. In the case of hydrocarbons such as gasoline or diesel fuel, however, syngas processing requires the addition of oxygen, followed by its removal which increases final product cost.
Methane, the predominant component of natural gas, although difficult to activate can be reacted with oxygen or oxygen-containing compounds such as water or carbon dioxide to produce syngas. This mixture is converted to syncrude with Fischer-Tropsch technology and then upgraded to transportation fuels using usual refining methods. Alternatively, syngas can be converted to liquid oxygenates which in turn can be converted to more conventional transportation fuels by catalysts such as certain zeolites.
Because syngas requires high capital investment, is produced in energy wasteful ways as by steam reforming where fuel is burned to supply heat of reforming, and it represents a detour to the production of hydrocarbons, other means of converting methane directly to higher hydrocarbons are needed.
Oxidative coupling has been recognized as a promising approach to the problem of methane conversion although the mechanism of action is not clear. In such processes, methane is contacted with solid materials referred to by various terms including catalysts, promoters, activators or contact materials Methane mixed with oxygen and catalyst is directly converted to ethane, ethylene, higher hydrocarbons and water. Carbon dioxide formation, which is highly favored thermodynamically, is undesirable as both oxygen and carbon are consumed without production of the desired higher value C.sub.2+ hydrocarbons.
Catalytic mixtures of yttrium-barium-copper oxides are highly active and 100% selective for producing CO.sub.2, that is, they are combustion catalysts. In order to obtain the required selectivity to hydrocarbon formation, Group IA metals, particularly lithium and sodium, were used in such catalytic mixtures. Under the conditions used for oxidative coupling, however, migration and loss of the alkali metal normally occurs. In order to avoid complete combustion most methods for oxidative conversion have been carried out in the absence of an oxygen-containing gas, relying on the oxygen theoretically being supplied by the catalyst Thus, the need for highly active, C.sub.2+ hydrocarbon selective and stable oxidative coupling promoters continues.
Many patents describe processes for converting methane to heavier hydrocarbons in the presence of reducible metal oxide catalysts Most of these patents require or imply the need for a separate stage to re-oxidize the catalyst. These include U.S. Pat. No. 4,444,984 which teaches a reducible oxide of tin as a catalyst; U.S. Pat. No. 4,495,374 disclosing the use of any reducible oxide promoted by an alkaline earth metal; 4,523,049 showing a reducible oxide catalyst promoted by an alkali or alkaline earth, requiring the presence of oxygen during the oxidative coupling reaction U.S. Pat. No. 4,656,155 specifies yttrium in a mixture requiring zirconium and alkali metal. U.S. Pat. No. 4,450,310 claims coupling promoted by alkaline earth oxides in the total absence of molecular oxygen. U.S. Pat. No. 4,482,644 teaches a barium-containing oxygen-deficient catalyst with a perovskite structure European Patent Application No. 198,251 covers a process conducted in the presence of free oxygen using a three component contact material of: (a) an oxide of calcium, strontium or barium, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin; (b) a sodium or potassium-containing material, and a Group IIA metal or a compound containing one, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin; (c) a Group IA metal compound, and optionally a material selected from the group consisting of chloride ions, compounds containing chloride ions, tin and compounds containing tin.
U.S. Pat. No. 3,885,020, although disclosing contact materials of the oxidative coupling type, is directed to a method of converting hydrocarbons to CO.sub.2 and water for pollution control. The combustion catalysts used consist of four components: (1) zirconium, tin or thorium; (2) an alkaline earth material; (3) a rare earth-type element such as scandium, lanthanum or cesium; and (4) a metal of the first transition series.
Baerns U.S. Pat. No. 4,608,449 relates to a methane conversion process using a suitable metal oxide catalyst, including tin oxide, on an oxide catalyst carrier carried out under temperatures of from 500.degree. C. to 900.degree. C. in the presence of oxygen at specified pressure.